Vulcanizable rubber composition for use as hot-water seals

ABSTRACT

This invention provides a vulcanizable rubber composition for use as hot-water seals comprising 100 parts by weight of a rubber mixture composed of 90 to 50% by weight of a nitrile-containing highly saturated copolymer rubber obtained by hydrogenating the conjugated diene portions of an unsaturated nitrile-conjugated diene copolymer rubber or unsaturated nitrile-conjugated diene-ethylenically unsaturated monomer terpolymer rubber, and 10 to 50% by weight of an ethylene-α-olefin copolymer rubber; 0.1 to 30 parts by weight of an organic peroxide vulcanizing agent; and optionally 10 to 100 parts by weight of carbon black. Seals formed by vulcanizing this composition show a very low percent change in volume and elongation, and are not liable to the release of carbon black or the production of surface cracks, upon exposure to hot water containing bleaching powder.

TECHNICAL FIELD

This invention relates to vulcanizable rubber compositions for use ashot-water seals.

BACKGROUND ART

It is known that compositions comprising a mixture of anitrile-containing highly saturated copolymer rubber obtained byhydrogenating the unsaturated bonds of the butadiene portions of anacrylonitrile-butadiene copolymer rubber, and an ethylene-propylenecopolymer rubber can yield vulcanized products having excellent weatherresistance and processability, having good heat resistance and oilresistance, and hence suitable for use as seals (for example, JapanesePatent Laid-Open Nos. 283639/'86 and 124951/'90).

Although the vulcanized products of these rubber compositions are rubbermaterials suitable for applications requiring excellent weatherresistance and processability in combination with high strength and oilresistance, especially for automobile parts and the like, much has beenunknown as to whether they can be used for applications other thanautomobile parts and the like. For example, it has been believed thatthey do not have satisfactory performance when used for applications inwhich they come into long-term contact with hot water containingbleaching powder capable of generating hypochlorous acid, such ashot-water seals used in hot-water generators for home or business use.

In such hot-water generators, ethylene-propylene copolymer rubber andacrylonitrile-butadiene copolymer rubber (NBR) have conventionally beenused as rubber materials for hot-water seals. However, in the case ofethylene-propylene copolymer rubber, its long-term use tends to causethe release of carbon black used as the reinforcing agent. Moreover, NBRhas the disadvantage that the rubber progressively hardens to produce alarge number of cracks in the rubber surface. Consequently, it has beendesired to develop an improved technique.

The present inventors have now discovered that, when a mixture of anitrile-containing highly saturated copolymer rubber andethylene-propylene copolymer rubber is vulcanized and then immersed inhot water containing bleaching powder for a long period of time, itschange in volume is unexpectedly slight. The present invention has beencompleted on the basis of this discovery.

DISCLOSURE OF THE INVENTION

According to the present invention, there is provided a vulcanizablerubber composition for use as hot-water seals comprising 100 parts byweight of a rubber mixture composed of 90 to 50% by weight of anitrile-containing highly saturated copolymer rubber obtained byhydrogenating the conjugated diene portions of an unsaturatednitrile-conjugated diene copolymer rubber or unsaturatednitrile-conjugated diene-ethylenically unsaturated monomer terpolymerrubber, and 10 to 50% by weight of an ethylene-α-olefin copolymer rubber(in which the sum of these components is 100% by weight); and 0.1 to 30parts by weight of an organic peroxide vulcanizing agent.

Nitrile-containing highly saturated copolymer rubber

The nitrile-containing highly saturated copolymer rubber used in thepresent invention is obtained by hydrogenating the conjugated dieneportions of an unsaturated nitrile-conjugated diene copolymer rubber orunsaturated nitrile-conjugated diene-ethylenically unsaturated monomerterpolymer rubber. This nitrile-containing highly saturated copolymerrubber has a bound acrylonitrile content of 10 to 50% by weight andpreferably 15 to 40% by weight, an iodine value of not greater than 80,preferably not greater than 60, and more preferably not greater than 30,and a Mooney viscosity (ML₁₊₄, 100° C.) of 30 to 300, preferably 50 to200, and more preferably 60 to 150. The bound acrylonitrile content ischosen so as to be most suitable for performance requirements. If theiodine value is greater than 80, the thermal resistance and strength ofthe resulting seals will be reduced. No particular limitation is placedon the lower limit of the iodine value. However, the iodine value shouldgenerally be at least 1 because unduly low iodine values may make itdifficult to vulcanize the rubber. If the Mooney viscosity is less than30, the resulting seals will have insufficient durability upon long-termexposure to high pressure and will show no improvement in compressionset and collapse (compressive relaxation). On the other hand, if theMooney viscosity is greater than 300, the unvulcanized rubber will havepoor processability during kneading.

Unsaturated nitrile-conjugated diene copolymer rubber

The monomers used to prepare the aforesaid unsaturatednitrile-conjugated diene copolymer rubber are illustrated by givingseveral examples. Specific examples of the unsaturated nitrile includeacrylonitrile, methacrylonitrile and α-chloroacrylonitrile; and specificexamples of the conjugated diene include 1,3-butadiene,2,3-dimethylbutadiene, isoprene and 1,3-pentadiene.

Unsaturated nitrile-conjugated diene-ethylenically unsaturated monomerterpolymer rubber

In order to prepared the unsaturated nitrile-conjugateddiene-ethylenically unsaturated monomer terpolymer rubber, anethylenically unsaturated monomer or other monomer copolymerizable withunsaturated nitriles and conjugated dienes is used.

Examples of such a monomer include aromatic vinyl monomer such asstyrene, α-methylstyrene and vinylpyridine; unconjugated diene monomerssuch as vinylnorbornene, dicyclopentadiene and 1,4-hexadiene; acrylatesand methacrylates having an alkyl group of about 1 to 18 carbon atoms,such as methyl acrylate, ethyl acrylate, propyl acrylate, n-butylacrylate, t-butyl acrylate, isobutyl acrylate, n-pentyl acrylate,isononyl acrylate, n-hexyl acrylate, 2-methylpentyl acrylate, n-octylacrylate, 2-ethylhexyl acrylate, n-dodecyl acrylate, methyl methacrylateand ethyl methacrylate; acrylates having an alkoxyalkyl group of about 2to 12 carbon atoms in total, such as methoxymethyl acrylate,methoxyethyl acrylate, ethoxyethyl acrylate, butoxyethyl acrylate andethoxypropyl acrylate; acrylates having a cyanoalkyl group of about 2 to12 carbon atoms, such as α- or β-cyanoethyl acrylate, α-, β- orγ-cyanopropyl acrylate, cyanobutyl acrylate, cyanohexyl acrylate andcyanooctyl acrylate; hydroxyalkyl-containing acrylates such as2-hydroxyethyl acrylate and hydroxypropyl acrylate; and unsaturateddicarboxylic acid mono- and dialkyl esters such as monoethyl maleate,dimethyl maleate, dimethyl fumarate, diethyl fumarate, di-n-butylfumarate, di-2-ethylhexyl fumarate, dimethyl itaconate, di-n-butylitaconate and di-2-ethylhexyl itaconate.

Other unsaturated carboxylic acid ester monomers include, for example,dimethylaminomethyl acrylate, diethylaminoethyl acrylate,3-(diethylamino)-2-hydroxypropyl acrylate and2,3-bis(difluoroamino)propyl acrylate. Further examples includefluoroalkyl-containing acrylates and methacrylates such astrifluoroethyl acrylate, tetrafluoropropyl acrylate, pentafluoropropylacrylate, heptafluorobutyl acrylate, octafluoropentyl acrylate,nonafluoropentyl acrylate, undecafluorohexyl acrylate,pentadecafluorooctyl acrylate, heptadecafluorononyl acrylate,heptadecafluorodecyl acrylate, nonadecafluorodecyl methacrylate,trifluoroethyl methacrylate, tetrafluoropropyl methacrylate,octafluoropentyl methacrylate, dodecafluoroheptyl methacrylate,pentadecafluorooctyl methacrylate and hexadecafluorononyl methacrylate;fluorinated benzyl acrylates and methacrylates such as fluorobenzylacrylate, fluorobenzyl methacrylate and difluorobenzyl methacrylate;fluoroalkyl vinyl ethers such as fluoroethyl vinyl ether, fluoropropylvinyl ether, trifluoromethyl vinyl ether, trifluoroethyl vinyl ether,perfluoropropyl vinyl ether and perfluorohexyl vinyl ether; andfluorine-containing vinyl monomers o- or p-trifluoromethylstyrene, vinylpentafluorobenzoate, difluoroethylene and tetrafluoroethylene, as wellas polyethylene glycol acrylate, polypropylene glycol acrylate, epoxyacrylate, urethane acrylate, polyethylene glycol methacrylate,polypropylene glycol methacrylate, epoxy methacrylate and urethanemethacrylate.

No particular limitation is placed on the amount in which theethylenically unsaturated monomer or other monomer copolymerizable withunsaturated nitrites and conjugated dienes is used. However, it isusually used in an amount of not greater than 80% by weight, preferably15 to 60% by weight, and more preferably 20 to 40% by weight, based onthe total amount of monomers.

Specific examples of the nitrile-containing highly saturated copolymerrubber include hydrogenated products of butadiene-acrylonitrilecopolymer rubber, isoprene-butadiene-acrylonitrile copolymer rubber andisoprene-acrylonitrile copolymer rubber; and hydrogenated products ofbutadiene-methyl acrylate-acrylonitrile copolymer rubber,butadiene-acrylic acid-acrylonitrile copolymer rubber andbutadiene-unsaturated dicarboxylic acid ester-acrylonitrile copolymerrubbers. A further example is butadiene-ethylene-acrylonitrile copolymerrubber.

No particular limitation is placed on the method for preparing thenitrile-containing highly saturated copolymer rubber used in the presentinvention, i.e., the method for hydrogenating the conjugated diene unitportions of an unsaturated nitrile-conjugated diene copolymer rubber orunsaturated nitrile-conjugated diene-ethylenically unsaturated monomerterpolymer rubber, and any conventional hydrogenation process may beemployed.

The catalysts which may be used for the purpose of hydrogenationinclude, for example, palladium/silica and palladium complexes (JapanesePatent Laid-Open No. 252405/'91). Moreover, rhodium and rutheniumcompounds as described in Japanese Patent Laid-Open Nos. 125858/'87,42937/'87, 45402/'89, 45403/'89, 45404/'89 and 45405/'89 may also beused.

Ethylene-α-olefin copolymer rubber

The ethylene-α-olefin copolymer rubber used in the present invention isa copolymer of ethylene and α-olefin or of these monomers and anunconjugated diene, and this is a substantially saturated copolymerrubber. Typical examples thereof are low-crystalline or non-crystallineelastomers consisting essentially of ethylene and an α-olefin of 3 to 14carbon atoms and having a degree of crystallinity of not greater than20% and preferably not greater than 10%, such as ethylene-propylenecopolymer rubber, ethylene-propylene-butene terpolymer rubber,ethylene-1-butene copolymer rubber, ethylene-propylene-unconjugateddiene terpolymer rubber, ethylene-propylene-1-butene-unconjugated dienecopolymer rubber, ethylene-1-butene-unconjugated diene multi-componentcopolymer rubber and mixtures thereof. Among them,ethylene-propylene-unconjugated diene terpolymer rubber is preferred.

Usable unconjugated dienes include dicyclopentadiene, 1,4-hexadiene,cyclooctadiene, methylene-norbornene, 5-ethylene-2-norbornene and thelike. Among them, dicyclopentadiene and 5-ethylene-2-norbornene arepreferred.

The ethylene-α-olefin copolymer rubber usually has a Mooney viscosity[ML₁₊₄ (100° C.)] of 10 to 180 and preferably 40 to 140, and preferablyhas an iodine value of not greater than 20.

In the above-described ethylene-α-olefin copolymer rubbers, the molarratio of ethylene units to α-olefin units is usually in the range of50/50 to 90/10 and preferably 60/40 to 84/16. In the case ofethylene-α-olefin-unconjugated diene copolymer rubbers (i.e., ternary ormulti-component copolymers), the molar ratio of (ethylene+α-olefin)units to unconjugated diene units is usually in the range of 98/2 to90/10 and preferably 97/3 to 94/6.

Mixture of nitrile-containing highly saturated copolymer rubber andethylene-α-olefin copolymer rubber

In the mixture of the nitrile-containing highly saturated copolymerrubber and the ethylene-α-olefin copolymer rubber which is used in thepresent invention, the mixing ratio of the rubber components is suchthat the nitrile-containing highly saturated copolymer rubber is usuallypresent in an amount of 90 to 50% by weight and preferably 80 to 60% byweight and the ethylene-α-olefin copolymer rubber is usually present inan amount of 10 to 50% by weight and preferably 20 to 40% by weight. Ifthe relative amount of the nitrile-containing highly saturated copolymerrubber is unduly large, the resulting seals will show a considerablechange in volume upon exposure to hot water. On the other hand, if therelative amount of the ethylene-α-olefin copolymer rubber is undulylarge, the resulting seals will be liable to the release of carbonblack.

Organic peroxide vulcanizing agent

The vulcanizable rubber compositions of the present invention needs tobe vulcanized with the aid of an organic peroxide vulcanizing agent. Theuse of a vulcanizing agent other than organic peroxide vulcanizingagents (in particular, a sulfur type vulcanizing agent) is undesirablein that the resulting seals will show a considerable change in volumeupon exposure to hot water. No particular limitation is placed on thetype of the organic peroxide vulcanizing agent used, and any of variousorganic peroxide vulcanizing agents commonly used for the peroxidevulcanization of rubber may be employed. Examples thereof includedicumyl peroxide, di-t-butyl peroxide, t-butylcumyl peroxide, benzoylperoxide,2,5-dimethyl-2,5-di(t-butylperoxy)hexyne-3,2,5-dimethyl-2,5-di(benzoylperoxy)hexyneand α,α'-bis(t-butylperoxy-m-isopropyl)benzene. Among them, di-t-butylperoxide is preferred. The organic peroxide vulcanizing agent is used inan amount of 0.1 to 30 parts by weight, preferably 0.5 to 20 parts byweight, and more preferably 1 to 10 parts by weight, per 100 parts byweight of the rubber mixture composed of the nitrile-containing highlysaturated copolymer rubber and the ethylene-α-olefin copolymer rubber.

Moreover, similarly to ordinary organic peroxide vulcanization, anunsaturated compound may also be used as a crosslinking aid in thepresent invention.

Examples thereof include ethylene glycol dimethacrylate,trimethylolpropane trimethacrylate, N,N'-m-phenylene-dimaleimide andtriallyl isocyanurate. Among others, trial lyl isocyanurate is preferredfrom the viewpoint of vulcanizate properties. The amount of crosslinkingaid added is in the range of 0.1 to 15 parts by weight per 100 parts byweight of the mixture composed of the nitrile-containing highlysaturated copolymer rubber and the ethylene-α-olefin copolymer rubber.

Carbon black

The vulcanizable rubber compositions of the present invention maycontain carbon black in order to reinforce hot-water seals formedtherefrom. Either high-structure or low-structure carbon black may beused, provided that it is selected from among various forms of carbonblack which are commonly used as reinforcing agents for rubber.Generally, individual particles of carbon black three-dimensionallygather in chains or clusters to form aggregates. Then, the term"structure" as used herein means the shape into which the particlesgather together, the number of particles constituting each aggregate,the magnitude of the interstitial volume, and the like.

The term "high-structure" refers to carbon black having an n-dibutylphthalate (DBP) oil absorption of not less than 118 ml per 100 g asmeasured according to the method for testing carbon black for use withrubber (JIS K6221-1982 or ASTM D2414-90).

The term "low-structure" refers to carbon black having an n-dibutylphthalate (DBP) oil absorption of not greater than 90 ml per 100 g asmeasured according to the method for testing carbon black for use withrubber (JIS K6221-1982 or ASTM D2414-90).

Specific examples of high-structure carbon black include HPC and CO forchannel black; ISAF-HS, HAF-HS, MAF, FEF, APF, CRF, CF, SCF and EOF forfurnace black; and HML for thermal black.

Specific examples of low-structure carbon black include EPO and MPC forchannel black; ISAF-LS, HAF-LS, MAF-LS, FEF-LS, FF, HMF, GPF and SRF forfurnace black; and MT and FT for thermal black.

These carbon blacks may be used alone or in admixture of two or more.Among others, low-structure carbon blacks such as FEF-LS, SRF, MAF-LSand MT are preferred. In hot-water seals formed by using them, therelease of carbon black is markedly reduced and its performance ismaintained even after long-term use.

Carbon black is used in an amount of 10 to 100 5 parts by weight andpreferably 20 to 80 parts by weight, per 100 parts by weight of themixture composed of the nitrile-containing highly saturated copolymerrubber and the ethylene-α-olefin copolymer rubber. If the amount ofcarbon black used is unduly small, the resulting vulcanized product willshow a reduction in hardness and sealing properties. On the other hand,if it is unduly large, a large amount of a plasticizer must be added inorder to regulate the hardness of the resulting hot-water seals.Consequently, the plasticizer may bleed to the surface and, moreover,the hot-water seals will show a reduction in strength.

If desired, the vulcanizable rubber compositions of the presentinvention may further contain other suitable compounding ingredientscommonly used in the field of the rubber industry, such as reinforcingagents (e.g., silica and talc), fillers (e.g., calcium carbonate andclay), processing aids, process oils, antioxidants and antiozonants.

Moreover, if desired, the vulcanizable rubber compositions of thepresent invention may further contain another rubber such as acrylicrubber, halogenated butyl rubber, fluororubber, acrylonitrile-butadienerubber, styrene-butadiene copolymer rubber, natural rubber orpolyisoprene rubber, or ethylene-vinyl acetate copolymer resin. Thisother rubber or resin is usually used in an amount of 5 to 20% by weightand preferably 5 to 15% by weight, based on the combined weight of thenitrile-containing highly saturated copolymer rubber, theethylene-α-olefin copolymer rubber and the other rubber or resin. Amongothers, the addition of acrylic rubber, halogenated butyl rubber orethylene-vinyl acetate copolymer resin is preferable in that theco-vulcanizability of the vulcanizable rubber composition is improvedthereby.

No particular limitation is placed on the method for preparing thevulcanizable rubber compositions of the present invention. However, theyare usually prepared by mixing and kneading the nitrile-containinghighly saturated copolymer rubber, the ethylene-α-olefin copolymerrubber, the vulcanizing agent and other compounding ingredients by meansof a mixing machine such as a roll mill or a Banbury mixer.

A mixture composed of the nitrile-containing highly saturated copolymerrubber and the ethylene-α-olefin copolymer rubber is usually prepared bya dry blending process in which the nitrile-containing highly saturatedcopolymer rubber and the ethylene-α-olefin copolymer rubber are mixed athigh temperature by means of a roll mill or a Banbury mixer, or a latexcoprecipitation process in which a latex of the nitrile-containinghighly saturated copolymer rubber is mixed with a latex of theethylene-α-olefin copolymer rubber, the resulting mixture is coagulatedand dried, and the resulting coagulum is heat-treated by means of anextruder or a Banbury mixer.

The vulcanized products obtained from the vulcanizable rubbercompositions of the present invention exhibit excellent performance whenused as hot-water seals for apparatus in which hot water containingbleaching powder capable of generating hypochlorous acid is hermeticallysealed for a long period of time, such as hot-water generators. Specificexamples of such hot-water seals include static seals (gaskets) such asO-rings and rectangular rings; and dynamic seals (packings) such asrotary seals, sliding seals and reciprocating seals.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention is more specifically described with reference tothe following examples. In the examples and comparative examples, allparts and percentages are by weight unless otherwise stated.

In each example, various tests were carried out according to theprocedures described below in detail.

(1) Vulcanizate properties

According to Japanese Industrial Standard (JIS) K6301, each of theunvulcanized rubber compositions prepared according to the formulationsshown in Table 1 was vulcanized at 170° C. for 20 minutes to obtain a 2mm thick sheet. Then, using a No. 3 dumbell die, specimens were punchedtherefrom and used to measure the tensile strength (kgf/cm²) andelongation (%) of the vulcanized product. Moreover, its hardness wasmeasured with a JIS spring-operated A type hardness tester.

(2) Heat aging

According to JIS K6301, the same vulcanized product as described abovewas maintained in air at 150° C. for 168 hours. Thereafter, its percentchange in tensile strength (%), percent change in elongation (%) andchange in hardness (points) were measured.

(3) Compression set

According to JIS K6301, the vulcanized product was maintained at 150° C.for 77 hours. Thereafter, its compression set (%) was measured.

(4) Immersion

An aqueous solution of sodium hypochlorite having a concentration of 200ppm was prepared by diluting a commercially available aqueous solution(5%) of sodium hypochlorite 25-fold with distilled water. The samevulcanized product as used in the above-described test for theevaluation of vulcanizate properties was immersed in this aqueoussolution at 80° C. After 168 hours, 336 hours, 500 hours and 1,000hours, its percent change in volume (%), percent change in tensilestrength (%), percent change in elongation (%) and change in hardness(points) were measured.

(5) Release of carbon black from the hot-water seal surface

The surface of the vulcanized product which had been immersed in anaqueous solution of sodium hypochlorite at 80° C. for 1,000 hoursaccording to the above-described test (4) was visually observed andevaluated on the following three-grade rating system.

3: No carbon black was released.

2: A slight amount of carbon black was released.

1: A large amount of carbon black was released.

(6) Production of cracks in the hot-water seal surface

The surface of the vulcanized product which had been immersed in anaqueous solution of sodium hypochlorite at 80° C. for 1,000 hoursaccording to the above-described test (4) was visually observed andevaluated on the following three-grade rating system.

3: No crack was produced.

2: A small number of cracks were produced.

1: A large number of cracks were produced.

EXAMPLES 1-7 AND COMPARATIVE EXAMPLES 1-3

According to the formulation shown in Table 1, a nitrile-containinghighly saturated copolymer rubber (Zetpol 1010, 2010L or 2000L), anethylene-α-olefin copolymer rubber (EPDM) and optionally ethylene-vinylacetate copolymer resin (ELVAX) were mixed. Subsequently, carbon black,a crosslinking aid (TAIC), an organic peroxide vulcanizing agent (Vulcap40KE) and an antioxidant (Nowguard 445) were incorporated therein. Thus,vulcanizable rubber compositions in accordance with the presentinvention were prepared. Then, the vulcanized products of thesecompositions were subjected to evaluation tests, and the results thusobtained are shown in Table 2 and Table 2 (contd.).

As comparative examples, vulcanizable rubber compositions containingacrylonitrile-butadiene copolymer rubber (Nipol 1042) or EPDM as shownin Table 1 were prepared and also subjected to the same evaluation testsas described above. The results thus obtained are shown in Table 2 andTable 2 (contd.).

                                      TABLE 1                                     __________________________________________________________________________                 Example No.                                                                   Example       Comparative Example                                Formulation  1 2 3 4 5 6 7 1  2   3                                           __________________________________________________________________________      Zetpol 1010 *1                                      70                        Zetpol 2010L *1                    80    70   80          70   70                                                          70                               Zetpol 2000L *1                                                     70                                         EPDM *2                            20                                          30   30    30    30   30   30   100                                               30                                      Nipol 1042 *3                                                                                                        100                                    ELVAX 40 *4                                   10                              FEF-LS carbon black                40    40   40    40              40                                         40    40    40                               FEF-HS carbon black                                       40                  MT carbon black                                                80                                              TAIC *5                            3                                           3    3     3     3    3    3    3                                           3                                             Vulcap 40KE *6                     8     8    8     8     8    8    8                                          8     8                                      Nowguard 445 *7                    1     1    1     1     1    1    1                                          1     1                                      Stearic acid                                                                                                               1                                Zinc oxide                                                                                                                 5                                Tetramethylthiuram disulfide                                                                                               1.5                              2-Mercaptobenzothiazole                                                                                                    0.5                              Sulfur                                                                                                                     0.5                            __________________________________________________________________________     Notes:                                                                        *1 Nitrilecontaining highly saturated copolymer rubbers manufactured by       Nippon Zeon Co., Ltd.                                                    

                            Bound acrylo-                                                          Iodine  Mooney     nitrile content                                            value   viscosity  (% by weight)                             __________________________________________________________________________      Zetpol 1010   10       85         44                                          Zetpol 2010L  4       58         36                                           Zetpol 2000L  4       65         36                                         __________________________________________________________________________     *2 An ethylene (55 mole %)propylene (40 mole %)dicyclopentadiene (5 mole      %) copolymer [Mooney viscosity = 80].                                         *3 An acrylonitrilebutadiene copolymer rubber manufactured by Nippon Zeon     Co., Ltd. [bound acrylonitrile content = 33.5% by weight].                    *4 Ethylenevinyl acetate copolymer resin manufactured by E. I. du Pont de     Nemours & Co. [vinyl acetate content = 40% by weight].                        *5 A crosslinking aid [triallyl isocyanurate].                                *6 An organic peroxide vulcanizing agent                                      [1,3bis(t-butyl-peroxyisopropyl)benzene].                                     *7 An antioxidant [substituted diphenylamine].                           

                                      TABLE 2                                     __________________________________________________________________________                 Example No.                                                                   Example              Comparative Example                         Measurement  1  2  3  4  5  6  7  1  2   3                                    __________________________________________________________________________    Vulcanizate properties                                                          Hardness (JIS A) 71 71 72 74 74 73 73 72 61 70                                Tensile strength (kgf/cm.sup.2)     233     218     205     228     254                                                  203      226    130     186                                                  172                                 Elongation (%)                        300     280     250     300                                                    250     230      280    120                                                   380     480                            Heat aging test                                                               Change in hardness (points)            6       5       5       5                                                     4       5        3      1                                                     24      13                             Percent change in tensile              -15  -15  -13  -14  -18  -16                                                  -8  -25      -48  -38                  strength (%)                                                                  Percent change in elonga-              -26  -23  -20  -21  -19  -18                                                  -9  -8   -76  -59                      tion (%)                                                                      Compression set                        24.3    23.6    22.4    22.8                                                  24.0    21.3     26.2   18.3                                                  38.2    88.9                         __________________________________________________________________________      168-hour immersion test                                                       Percent change in volume (%)   5.5     4.2     3.3     3.1     4.8                                                   2.5      6.0    1.0     8.5                                                   8.4                                    Change in hardness (points)     -4   -3   -2   -2   -4   -2    -4  0                                                    -2   -4                             Percent change in tensile       -8   -10  -14  -9   -12  -8    -5  -49                                               -23  -10                               strength (%)                                                                  Percent change in elongation   -5   -6   -8   -5   -7   -5    -3  -8                                                 -18  -12                               (%)                                                                           336-hour immersion test                                                       Percent change in volume (%)   5.9     4.6     3.5     3.2     5.0                                                   3.2      6.4    1.0     12.4                                                  10.2                                   Change in hardness (points)    -4   -3   -2   -3   -6   -3    -4  2                                                    4       -1                           Percent change in tensile     -10  -12  -20  -9   -15  -8    -8  -63                                                 -28  -18                               strength (%)                                                                  Percent change in elongation    -5   -7   -16  -7   -8   -6    -5  -42                                               -26  -26                               (%)                                                                           500-hour immersion test                                                       Percent change in volume (%)   7.1     5.9     5.2     4.2     6.8                                                   4.0      6.8    1.8     13.6                                                  10.5                                   Change in hardness (points)     -5   -3   0       -1   -5   -3    -4  3                                                    8       3                        Percent change in tensile   -10  -13  -16  -10  -18  -12   -9  -61  -31                                               -36                                   strength (%)                                                                  Percent change in elongation   -6   -6   -8   -8   -15  -9    -8  -33                                                -38  -48                               (%)                                                                           1,000-hour immersion test                                                     Percent change in volume (%)  6.5     5.4     4.3     3.9     6.5                                                    3.8      6.1    0.4     15.2                                                  10.0                                   Change in hardness (points)            -4   -3   0       0       -5                                                  -1    -2  4       15      6                                                    Percent change in tensile                                                           -12   -15  -24  -12  -22                                               -14   -10 -65  -38  -51                strength (%)                                                                  Percent change in elongation           -6   -8   -12  -9   -19  -12                                                  -8  -42  -49  -82                      (%)                                                                           Separation of carbon black       3       3       3       3       2                                                    3        3      1       3                                                    2                                      Surface cracking                 3       3       3       3       3                                                    3        3      2       1                                                    3                                    __________________________________________________________________________

It can be seen from the results shown in Table 2 and Table 2 (contd.)that the vulcanized products obtained by vulcanizing the rubbercompositions of the present invention show a low percent change involume and elongation when subjected to long-term immersion tests, thusexhibiting excellent performance upon exposure to hot water generatinghypochlorous acid. It can also be seen that the release of carbon blackand the production of surface cracks are markedly decreased as comparedwith conventionally used EPDM and NBR.

Moreover, it can be seen that, especially when low-structure carbonblack (FEF-LS) was used (Examples 1, 2, 3, 4, 6 and 7), the separabilityof carbon black is further improved as compared with the example inwhich high-structure carbon black (FEF-HS) was used (Example 5).

Furthermore, it can be seen that, as compared with the example in whicha sulfur type vulcanizing agent was used (Comparative Example 3), theaforesaid vulcanized products show a much lower percent change in volumewhen subjected to immersion tests and a much lower percent change inhardness when subjected to a heat aging test.

Industrial Applicability

As illustrated by the above-described examples and comparative examples,the hot-water seals obtained by vulcanizing the rubber compositions ofthe present invention show a much lower percent change in volume andelongation, and are less liable to the release of carbon black and theproduction of surface cracks, as compared with conventional hot-waterseals formed from NBR and the like. Accordingly, the vulcanizable rubbercompositions of the present invention can be used as materials forhot-water seals to industrial advantage.

We claim:
 1. A vulcanizable rubber composition for use as hot-waterseals comprising 100 parts by weight of a rubber mixture composed of 90to 50% by weight of a nitrile-containing highly saturated copolymerrubber having an iodine value of not greater than 80 obtained byhydrogenating the conjugated diene portions of an unsaturatednitrile-conjugated diene copolymer rubber of unsaturatednitrile-conjugated diene-ethylenically unsaturated monomer terpolymerrubber, and 10 to 50% by weight of an ethylene-α-olefin copolymerrubber, wherein the sum of these components is 100% by weight; and 0.1to 30 parts by weight of an organic peroxide vulcanizing agent.
 2. Avulcanizable rubber composition for use as hot-water seals as claimed inclaim 1 wherein the unsaturated nitrile in the unsaturatednitrile-conjugated diene copolymer rubber or unsaturatednitrile-conjugated diene-ethylenically unsaturated monomer terpolymerrubber is selected from the group consisting of acrylonitrile,methacrylonitrile and α-chloroacrylonitrile.
 3. A vulcanizable rubbercomposition for use as hot-water seals as claimed in claim 1 wherein theconjugated diene in the unsaturated nitrile-conjugated diene copolymerrubber or unsaturated nitrile-conjugated diene-ethylenically unsaturatedmonomer terpolymer rubber is selected from the group consisting of1,3-butadiene, 2,3-dimethylbutadiene, isoprene and 1,3-pentadiene.
 4. Avulcanizable rubber composition for use as hot-water seals as claimed inclaim 1 wherein the unsaturated nitrile-conjugated diene copolymerrubber or unsaturated nitrile-conjugated diene-ethylenically unsaturatedmonomer terpolymer rubber is an unsaturated nitrile-conjugateddiene-unsaturated dicarboxylic acid ester terpolymer rubber.
 5. Avulcanizable rubber composition for use as hot-water seals as claimed inclaim 1 wherein the unsaturated nitrile-conjugated diene-ethylenicallyunsaturated monomer terpolymer rubber has 10 to 50% by weight ofunsaturated nitrile units and 1 to 80% by weight of unsaturateddicarboxylic acid ester units.
 6. A vulcanizable rubber composition foruse as hot-water seals as claimed in claim 1 wherein theethylene-α-olefin copolymer rubber is an ethylene-propylene-unconjugateddiene terpolymer rubber.
 7. A vulcanizable rubber composition for use ashot-water seals as claimed in claim 1 wherein the unconjugated diene inthe ethylene-propylene-unconjugated diene terpolymer rubber isdicyclopentadiene.
 8. A vulcanizable rubber composition for use ashot-water seals as claimed in claim 1 which contains carbon black.
 9. Avulcanizable rubber composition for use as hot-water seals as claimed inclaim 8 wherein the carbon black is low-structure carbon black.
 10. Avulcanizable rubber composition for use as hot-water seals as claimed inclaim 9 wherein the low-structure carbon black is selected from thegroup consisting of FEF-LS, MAF-LS, SRF and MT.
 11. A method of sealinghot-water generators which comprises providing said hot water generatorswith hot-water seals comprising a vulcanizable rubber compositioncomprising 100 parts by weight of a rubber mixture composed of 90 to 50%by weight of a nitrile-containing saturated copolymer rubber having aniodine value of not greater than 80 obtained by hydrogenating theconjugated diene portions of an unsaturated nitrile-conjugated dienecopolymer rubber or unsaturated nitrile-conjugated diene-ethylenicallyunsaturated monomer terpolymer rubber, and 10 to 50% by weight of anethylene-α-olefin copolymer rubber, wherein the sum of these componentsis 100% by weight; and 0.1 to 30 parts by weight of an organic peroxidevulcanizing agent.
 12. The method of claim 11 of sealing hot watergenerators wherein the unsaturated nitrile in the unsaturatednitrile-conjugated diene copolymer rubber or unsaturatednitrile-conjugated diene-ethylenically unsaturated monomer terpolymerrubber is selected from the group consisting of acrylonitrile,methacrylonitrile and α-chloroacrylonitrile.
 13. The method of claim 11of sealing hot water generators wherein the conjugated diene in theunsaturated nitrile-conjugated diene copolymer rubber or unsaturatednitrile-conjugated diene-ethylenically unsaturated monomer terpolymerrubber is selected from the group consisting of 1,3-butadiene,2,3-dimethylbutadiene, isoprene and 1,3-pentadiene.
 14. The method ofclaim 11 of sealing hot water generators wherein the unsaturatednitrile-conjugated diene copolymer rubber or unsaturatednitrile-conjugated diene-ethylenically unsaturated monomer terpolymerrubber is an unsaturated nitrile-conjugated diene-unsaturateddicarboxylic acid ester terpolymer rubber.
 15. The method of claim 11 ofsealing hot water generators wherein the unsaturated nitrile-conjugateddiene-ethylenically unsaturated monomer terpolymer rubber has 10 to 50%by weight of unsaturated nitrile units and 1 to 80% by weight ofunsaturated dicarboxylic acid ester units.
 16. The method of claim 11 ofsealing hot water generators wherein the ethylene-α-olefin copolymerrubber is an ethylene-propylene-unconjugated diene terpolymer rubber.17. The method of claim 11 of sealing hot water generators wherein theunconjugated diene in the ethylene-propylene-unconjugated dieneterpolymer rubber is dicyclopentadiene.
 18. The method of claim 11 ofsealing hot water generators wherein the hot water generators containhot water and bleaching powder capable of generating hypochlorous acid.